Added Longest Palindromic Substring problem in Java under strings

Author: Nandan P Aghera

Java/strings/LongestPalindromicSubstring.java

@@ -0,0 +1,49 @@
+// Problem: Longest Palindromic Substring
+// Language: Java
+// Description: Finds the longest palindromic substring in a given string. 
+// Handles all edge cases (empty string, single character, multiple solutions).
+
+import java.util.*;
+
+public class LongestPalindromicSubstring {
+
+    // Method to find the longest palindromic substring
+    public static String longestPalindrome(String s) {
+        if (s == null || s.length() < 1) return "";
+
+        int start = 0, end = 0; // indices of the longest palindrome
+        for (int i = 0; i < s.length(); i++) {
+            int len1 = expandAroundCenter(s, i, i);       // odd length palindrome
+            int len2 = expandAroundCenter(s, i, i + 1);   // even length palindrome
+            int len = Math.max(len1, len2);
+            if (len > end - start) {
+                start = i - (len - 1) / 2;
+                end = i + len / 2;
+            }
+        }
+        return s.substring(start, end + 1);
+    }
+
+    // Helper method to expand palindrome around center
+    private static int expandAroundCenter(String s, int left, int right) {
+        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
+            left--;
+            right++;
+        }
+        return right - left - 1; // length of palindrome
+    }
+
+    // Main method to take input and display output
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        System.out.print("Enter a string: ");
+        String input = sc.nextLine();
+
+        String result = longestPalindrome(input);
+
+        System.out.println("\nLongest Palindromic Substring: " + result);
+        System.out.println("Length: " + result.length());
+
+        sc.close();
+    }
+}